Adds an individual action to this.
This action will only be activated if the given button/ring/strip number in
index is interacted while the current mode is
mode. -1 may be used for simple cases, so the action is triggered on all modes.
label should be considered user-visible, so internationalization rules apply. Some windowing systems may be able to use
those for user feedback.
the type of pad feature that will trigger this action
the 0-indexed button/ring/strip number that will trigger this action
the mode that will trigger this action, or -1 for all modes.
Human readable description of this action, this string should be deemed user-visible.
action name that will be activated in the `GActionGroup`